In a known general structure of an organic electroluminescence element (hereinafter referred to as “organic EL element”), an anode made of a transparent electrode, a hole transport layer, a light emitting layer, an electron injection layer, and a cathode are stacked on a surface of a transparent substrate in this order. It is known that such an organic EL element is used to produce a planar light emitting device (lighting panel). In this organic EL element, light is produced in an organic light emitting layer in response to application of voltage between the anode and the cathode, and the produced light is emitted outside through the transparent electrode and the transparent substrate and goes outside.
The organic EL element gives a self-emission light in various wavelengths, with a relatively high yield. Such organic EL elements are expected to be applied for production of displaying apparatuses (e.g., light emitters used for such as flat panel displays), and light sources (e.g., liquid-crystal displaying backlights and illuminating light sources). Some of organic EL elements have already been developed for practical uses. Recently, in consideration of application and development of organic EL elements to such uses, an organic EL element having high efficiency, prolonged lifetime, and high brightness is expected.
It is considered that the efficiency of the organic EL element is mainly dominated by three of electrical optical conversion efficiency, driving voltage, and light-outcoupling efficiency. With regard to the electrical optical conversion efficiency, it was reported that the organic EL element with the light emitting layer made of phosphorescent light emitting material can have external quantum efficiency greater than 20%. The external quantum efficiency of 20% is considered to be corresponding to internal quantum efficiency of about 100%. It is considered that the organic EL element having the electrical optical conversion efficiency reaching a limiting value has been developed in the experimental stage. In view of the driving voltage, an organic EL element which shows relatively high brightness in receipt of voltage higher by 10 to 20% than voltage corresponding to an energy gap of the light emitting layer has been developed. In other words, improvement of efficiency of the organic EL element resulting from a decrease in voltage is not so effective. Consequently, it is expected that improvement of these two factors is not so effective for an increase in the efficiency of the organic EL element.
Generally, the organic EL element has the light-outcoupling efficiency in the range of about 20 to 30% (this value slightly changes depending on lighting patterns, and/or a layer structure between the anode and the cathode). This light-outcoupling efficiency is not high. This low light-outcoupling efficiency may be explained by the following consideration: materials used for light emitting portion and a vicinity thereof have characteristics such as a high refractive index and light absorption properties, and therefore the total reflection at the interfaces between materials with different refractive indices and absorption of light by materials may occur and this causes inhibition of effective propagation of light to the outside. Such low light-outcoupling efficiency means 70 to 80% of the total amount of produced light does not effectively contribute to light emission. Consequently, it is considered that improvement of the light-outcoupling efficiency causes a great increase in the efficiency of the organic EL element.
In consideration of the above background, there is studied and developed actively to improve the light-outcoupling efficiency. Especially, there have been many efforts to increase the amount of light which is produced in the organic layer and reaches the substrate. For example, the organic layer has the refractive index of about 1.7, and a glass layer generally serving as the substrate has the refractive index of about 1.5. In this case, a loss caused by total reflection at the interface between the organic layer and the glass layer probably reaches about 50% of total amount of radiated light. The value of about 50% is calculated by use of point source approximation in consideration that the emitted light is expressed as an integration of three dimensional radiation of light from organic molecules. Unfortunately, the total reflection at the interface between the organic layer and the substrate tends to cause a great loss. In view of this, it is possible to greatly improve the light-outcoupling efficiency of the organic EL element by decreasing the loss caused by the total reflection between the organic layer and the substrate.